Role of Droplet Dynamics on Contact Line Depinning in Shearing Gas Flow

Sessile droplets exposed to shearing gas flows resist depinning owing to surface tension and contact angle hysteresis. It is known that contact line depinning occurs when the shearing gas flow is large enough to deform the droplet beyond its contact angle hysteresis. This work explores the contact line depinning process by visualizing growing droplets on a porous layer in laminar shear gas flows. High-speed imaging of droplets revealed an oscillatory motion in droplets, which is speculated to originate from an interaction between the drag force and surface tension effects. This oscillatory motion creates an inertial force within the droplet which combines with the drag force when droplet acceleration is in the stream-wise direction. The combined effect competes against the droplet adhesion force, setting the depinning criteria. Analyzing droplet images revealed that droplet local velocity and acceleration (i.e., sessile droplet dynamics prior to detachment from the substrate) increase with the superficial gas velocity. At the same time, the contact line depinning occurs at a smaller droplet size for higher superficial gas velocities. This results in a "hill-like" variation of the inertial force as a function of the convective Weber number, We_conv, causing a local maximum in the inertial force data (We_conv scales the inertia effects of the shear flow to surface tension effects). For the experimental condition tested in the current study, the inertial force created in the droplet could reach up to half of the adhesion force, making the drag force only responsible for the other half to depin the droplet contact line. Even at low superficial gas velocities, which featured lower droplet oscillations, the inertial force created in the droplet was considerable with respect to the adhesion force, reaching around one-third of the adhesion force.